In front of you are two boxes.
Box A contains $1000.
If Omega predicted you would just take Box B, Box B contains $1,000,000.
If Omega predicted you would take both boxes, Box B contains nothing. Omega will also go back in time and kill your grandfather.
Nobody has any idea how accurate Omega's predictions are.
Does the fact that you exist prove that Omega thought you would just take one box, so Box B definitely contains the million dollars, and if you took both boxes it would imply that Omega was a terrible predictor so you might as well just take both boxes and run?
It's reasonable to note when a premise is ridiculous. That said, what would you do if the premise was true?
I think I disagree with most of that; I think most of "the opposition" (not that I much like that framing) have views that are poorly defined, borderline-incoherent, or both.
(Not because "the opposition" are idiots; I suspect it's also true that most atheists have rather fuzzy notions of just what "the existence of God" entails.)
Sure, it's possible (at least, I think it is) to formulate a precisely-defined definition of "God" and debate whether there is any such entity. But that hardly ever happens; when people argue about the topic, what usually happens is that everyone has their own idiosyncratic and tactically-varying idea of what "God" and "God exists" means.
And I don't think shminux introduced the notion of God in order to make fun of it; I think he thought it was a useful analogy to explain how he feels about the idea of time travel. Something like "it's a thing lots of people assume is a reasonable idea that one ought to consider the possibility of, but I think it actually doesn't make sense, and the more closely you look at it the less sense it makes, and bringing it into a discussion usually makes that discussion less useful rather than more".